The ingestion of water in a healthy individual does not lead to hyponatremia as the antidiuretic hormone (ADH) gets suppressed, allowing the kidney to excrete the excess water. On the other hand, when an individual becomes dehydrated with hypovolemia, the ADH gets released, allowing the kidney to preserve as much water as possible. The diagnosis of hyponatremia in the ICU can be made easy in a systematic way by answering these four main questions:
Is it real hyponatremia?
High serum osmolality associated with hyponatremia results from the addition of exogenous effective osmoles like mannitol, sucrose, maltose, sorbitol, glycine, or radiocontrast. These compounds can cause an increased serum osmolality and tonicity leading to hyponatremia as water moves outside the cells diluting extracellular compartment.
Both urea and ethanol (ETOH) or other alcohols (if present) are considered ineffective osmoles as they equilibrate across cell membranes and cause little effect on water movement. However, they cause an elevation in the measured serum osmolality (but not the serum tonicity). If this is not recognized by the clinician, then patients with azotemia and hyponatremia can be diagnosed as having hypertonic or isotonic hyponatremia instead of hypotonic hyponatremia.
In cases of hyperlipidemia and hyperproteinemia, the sodium concentration will be low in the plasma containing the lipid or protein but normal in the water phase of the plasma. As autoanalyzers measure the sodium in the plasma phase (not the water phase), thus, sodium concentration will falsely be low (psudohyponatremia).
Is water excretion appropriate?
In cases of psychogenic polydipsia or individuals who are on a high fluid, low protein diet, the water excretion will be normal and the urine will be very diluted with a urine osmolality of less than 100 mOsm/kg. If the urine osmolality is elevated, this means that the ADH is excreted and the next step is to determine if that release is appropriate or not.
Is ADH secretion appropriate?
The main stimuli for ADH are high serum osmolality and hypovolemia. ADH excretion is appropriate in cases of volume contraction. The serum sodium level depends on the concentration of sodium in the lost fluid. Loss of fluid that has a high sodium concentration causes hyponatremia with hypovolemia. In cases of volume excess such as congestive heart failure, the ADH will maladapt to the situation. In these situations, sodium content can be normal or high but the water volume is increased causing hyponatremia. Secretion of ADH is inappropriate SIADH (and other disorders). This leads to impaired water excretion caused by the inability to suppress the ADH. If water intake exceeds the reduced urine output, the ensuing water retention leads to the development of hyponatremia.
What is the urine sodium concertation?
The sodium urine concentration helps in determining renal from extrarenal volume loss in hypovolemic hyponatremia. High urine sodium support the diagnosis of SIADH in euvolemic hyponatremia. Other differentials are included in the infographic. in patients with hypervolemia, urine sodium can differentiate renal failure from heart failure, nephrotic syndrome, or cirrhosis.